Comparison of fluid and particle-in-cell 3D simulations of negative streamers in CO2 with admixtures of C4F7N

CO2 with an admixture of C4F7N could serve as an eco-friendly alternative to the extreme greenhouse gas SF6 in high-voltage insulation. Streamer discharges in such gases are different from those in air due to the rapid conductivity decay in the streamer channels. Furthermore, since no effective photoionisation mechanism is known, we expect discharge growth to be more stochastic than in air. In this paper we investigate whether conventional fluid models provide an good approximation to a particle-in-cell model for negative streamers in CO2 with admixtures of 1 or 10% C4F7N Higher fractions were not included, as C4F7N admixtures in high-voltage insulation rarely exceed 10% C4F7N. We focus on 3D simulations of negative streamers. First we review cross section databases for C4F7N and CO2. Then we compare a two-term Boltzmann solver with a Monte Carlo method to compute reaction and transport coefficients from the cross sections. Afterwards we compare 3D fluid simulations with the local field (LFA) or local energy approximation (LEA) against particle simulations. In general, we find that the results of particle and fluid models are quite similar. One difference we observe is that particle simulations are intrinsically stochastic, leading to more branching. Furthermore, the LEA model does not show better agreement with the particle simulations than the LFA model. We also discuss the effect and choice of different boundary conditions on the negative rod electrode.

• View at Publisher

Free Full Text ( Final Version , 3mb )